Amplifier circuit



Dec.' 2s, 1937.

Fic.

E. PETERSON l AMPLIFIER CIRCUIT ,Filed `April 1. 195e CLA SS B /NVEA/TOR E. PE'TERSU/V ATTORNEY Patented Dec. 28, 1937 *A AMPLIFIERCIRCUIT Eugene Peterson, New York, N. Y., assigner to Bell TelephoneLaboratories, Incorporated, New York, N. lY., a corporation of New YorkApplication April 1, 1936, serial No. 72,145

2 Claims.

ample, to control transmission efficiency, plate power efiiciency ormodulation or distortion in Such systems.

It is also an object of the invention to facilil tate application offeedback in such systems.

Pronounced difficulty has been encountered in cascade operation ofvacuum tubes whose grids are driven down to or below the plate currentextinction poin't, due to marked increase of gain l5 ofv the circuitwith load or input level, an effect which for convenience will bereferred to 'as gasping effect; and it has been found that the gaspingeffect can be reduced or eliminated by A proper feedback in thecircuit.`

A specific feature of the invention is an amplifier having cascadedtubes operated with space current interruptions, and having connectionsfor feeding waves back in the amplifier in gain-reducing phase and inmagnitude sufficient to render the gasping effect less than withoutfeedback, The tubes may be operated, for example, in accordance withso-called 'class B operation, in which the steady grid potential withoutalternating current input is such that the space current withoutfeedback would be extinguished for the order of half the period of thewaves to be amplified. The amplifier stages may be, for example, of thebalanced or push-pull type, with the potentials of the control grids ofthe output stage maintained always positive during operation of theamplifier and the potentials of the control grids of the preceding tubesmaintained always negative during operation of the amplifier.Maintaining the grid potential for the out- "40 put stage alwayspositive serves to avoid change of sign of the grid potential that mightchange the input impedance of the stage sufficiently to causeobjectionable changein its amplification. Maintaining the grids of thepreceding stages negative facilitates driving them with stages of marywinding of an output transformer of the push-pull stage to the grid ofthe single-sided stage. VThe push-pull stage maybe, for example, a classB stage; and the cascade amplifier may u be formed of two single-sidedamplifiers with the feedback connection crossed from the output of oneto the input of the other, in the case, for example, of an even numberof stages in the feedback loop, or with the feedback connection linkingthe output and input of one of the singlesided amplifiers in the case,for example, of an odd number of stages in the feedback loop. Each 0fthese simple types of feedback connections have been found effective inproviding reverse feedback rendering linearity and gain stability cf theampliiier greater than without feedback, with the push-pull stage eithera class A stage or a stage operated with space current interruptions, asfor instance, a class B stage.

Other objects and aspects of the invention will be apparent from thefollowing description and claims.

Figs. 1, 2, 3 and 4 are circuit diagrams of four forms of the invention.

Fig. 1 shows a two-stage push-pull amplifier for amplifying wavesreceived from circuit I and transmitting the amplified waves to circuit2. The waves may be, for example, speech waves or a broad band ofcarrier waves transmitting a number of speech messages by multiplexcarrier telephony. The first stage of the amplifier comprises twosimilar vacuum tubes 3 and 3', shown by way of example as triodes. Thesecond stage may be a power stage. It comprises two tubes il and liwhich are also alike and which are also shown, by way of` example, astriodes. Y

i and cathode of tube 3 being connected to receive the Voltage acrossarm 11.

The fourI ratio arms of bridge l l likewise comprise foui resistanoesri, Ieri, Ici' and r, the grid and cathode of tube 3' being Iconnectedto receive the voltage across this resistance r1.

A plate current supply source 25 is shown lfor tubes l and 4'. IThetubes may be operated inK accordance with so-called class B operation,in which the grid potential in the absence of alternating current inputis in the neighborhood of the value required to reduce the space currentsubstantially to zero. For example, the grids of tubes 4 and 4 are shownconnected to the cathi odes through high resistances 28 and 28',respectively, shunted by choke coils 29 and 29' of low impedance fordirect current.

The plate of tube 3 is connected to the grid of tube 4 through astopping condenser 21; and

the plate of tube 3' is connected to the grid of tube 4 through astopping condenser 21'.

A plate current supply source 42 supplies direct space current for thetubes 3 and 3 through choke coils 3I and 32, respectively, which areclosely coupled. (Sources 42 and Z5 may be common if desired.)

Negative grid bias for the tubes 3 and 3- is supplied by a voltagesource 46 which may condition the tubes for class B operation andmaintain their grids always negative during operation of the amplifier.

As indicated above, pronounced difficulty has been encountered in classB operation of cascaded stages as in Fig. 1, due to gasping effect, butthis effect can be Vreduced or eliminated by negative feedback. Theproper feedback is provided in Fig. 1 by a feedback path comprising twofeedback connections I6 and Il, each including a stopping condenser I9.

The alternating current potentials of the plate of tube 4 and the gridof tube 3 are approximately opposite in phase; and the alternatingcurrent potentials of the plate of tube 4 and the grid of tube 3 areapproximately opposite in phase.

With the two-stage balanced amplifier, the feedback is obtained in thedesired phase by reversing the feedback leads I6 and I'l, i. e.,crossing them over from one side of the push-pull circuit to the otherside, as disclosed in my Patent 1,955,827, April 24, 1934. Thus, lead I6is connected from the plate of tube 4 (which is in the upper side of thepush-pull circuit) to the bridge circuit I I in the input circuit oftube 3 (which is in the lower side of the push-pull circuit); and leadIl' is similarly connected between the plate of tube 4 (which is in thelower side of the balanced circuit) to the input bridge circuit I I oftube 3 (which is in the upper side of the balanced circuit). The bridgecircuits Il and II render the feedback path comprising connections I6and Il conjugate to windings I2 and I2 and circuit I at balance of thebridges, as pointed out in my above-mentioned patent, for example. Thegainreduction effected by the feedback may be large. The gain of theamplifier without feedback should then correspondingly exceed the gaindesired with feedback. For instance, the gain without feedback may be ofthe order or forty decibels and the feedback may reduce the gain to,say, a value of the order of twenty decibels. Feedback resistors 56 areshown in leads I5 and II for controlling the amount of feedback.

The close coupling of coils 3| and 32, and likewise the close couplingof coils 8 and 8', reduces parallel-singing tendency of the amplifier aspointed out in my above-mentioned patent.

Fig. 2 shows an amplier circuit somewhat similar to that of Fig. 1.However, in Fig. 2 the first stage is a single-sided stage which may bea class A operated tube 3, and a stage shown as a single-sided stagecomprising a tube 3A which may also be a class A stage, is interposedbetween tube 3 and the output stage, which may be like the output stageof Fig. 1. Moreover, negative feedback, which may serve to increaselinearity and gain stability of the amplifier, is provided by a feedbackconnection extending through condenser I9 and resistor 56 and linkingthe plate of tube 4 and the grid of tube 3. The tube 3 is connected tocircuit l by an input transformer IIO which has a secondary winding I2associated with input bridge II as in Fig. 1. If desired, the stagedriving the output stage may be a push-pull class B stage of the typeshown as the first stage of Fig. 1.

Fig. 3 shows an amplifier circuit somewhat similar to that of Fig. 2with the second stage of Fig. 2 omitted. However, in Fig. 3 the class Bpush-pull output stage has a grid biasing battery |22 which may maintainthe grids always negative during operation of the amplifier; and in Fig.3, negativev feedback, which may reduce the amplifier gain several timesten decibels and serve to increase linearity and gain stability of theamplifier, is provided by a feedback connection I1 linking the plate oftube 4 and the grid of tube 3.

Fig. 4 shows an amplier circuit somewhat like that ofV Fig. 3, but'withthe last stage having its grids always positive, as in the case of thelast stage of Fig. 1, and with two stages interposed between the firststage and the output stage. The first ofV these interposed stages may bea class A stage. It may be either a single-sided stage such' as thatshown as the second stage of Fig. 2, or a push-pull stage, and is shownby way of example as a push-pull stage comprising tubes 3A and 3A. Thesecond of these interposed stages, i. e., the third stage of theamplifier, comprises tubes 3B and 3B and is a push-pull, class B stagewith its grids maintained always negative as in the case of the firststage of Fig. 1. Negative feedback is provided by feedback connectionIl. This feedback may reduce the gain of the amplifier several times tendecibels, for example, and serve to reduce or eliminate gasping effectdue to the cascaded class B stages of the amplifier.

What is claimed is:

1. In combination, an amplifier having cascaded stages, each' of saidstages comprising space discharge devices in push-pull relation operatedin accordance with so-called class B operation, in which the steadycontrol-grid potential of each of the devices is such that the spacecurrent of the device is interrupted for the order of half the period ofthe waves to be amplified, said class B operation of said cascaded.stages producing an objectionable gasping effect consisting of increaseof gain of said amplifier with increase of load, and means for improvingoperation of the amplifier comprising connections for feeding back tothe first of said cascaded stages a portion of the output waves of theamplifier in gain-reducing phase and in amount sufficient to suppresssaid gasping effect below the value without feedback.

2. In combination, an amplifier having cascaded stages, each of saidstages comprising space discharge devices in push-pull relation operatedin accordance with so-called class B operation, in which the steadycontrol-grid potential of each of the devices is such -that the spacecurrent of. the device is interrupted for the order of half the periodof the waves to be amplified, the last of said stages having means formaintaining its control grids always positive during operation of theamplifier and the preceding stage having means for maintaining itscontrol grids always negative during operation of the amplifier, andmeans for improving operation of the amplifier comprising connectionsfor feeding back to the first of said cascaded stages a portion oftheoutput waves of the amplifier in gain-reducing phase and in amountsufficient to reduce the distortion below the distortion level withoutfeedback.

EUGENE PETERSON.

